A WordPress child theme is a theme that inherits the functionality of another theme, called the parent theme, and allows you to modify, or add to, the functionality of that parent theme. The child theme article on the WordPress Codex shows how to create a basic child theme and explains what you can do with it.
Making a child theme is very simple. Create a directory by using FTP or whatever file management application your host provides. Put a properly formatted style.css file in it, and you have a child theme! The child theme contains a “Template: parent-theme-dir” header to distinguish it from standalone themes. With a little understanding of HTML and CSS, you can make that very basic child theme modify the styling and layout of a parent theme to any extent without editing the files of the parent theme itself. That way, when the parent theme is updated, your modifications are preserved.
For this reason, child themes are the recommended way of making modifications to a theme.
Related Reading:
How to Modify Your Theme the Right Way
